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Ultrasound
Learning Objectives:
• Describe the properties of ultrasound;
• Describe the piezoelectric effect;
• Explain how ultrasound transducers emit and receive high-frequency sound;
• Describe the principles of ultrasound scanning;
• Describe the difference between A-scan and B-scan;
• Calculate the acoustic impedance using the equation Z = ρc;
• Calculate the fraction of reflected intensity
• Describe the importance of impedance matching;
• Explain why a gel is required for effective ultrasound imaging techniques.
• Explain what is meant by the Doppler effect;
• Explain qualitatively how the Doppler effect can be used to determine the speed of blood.
What is Ultrasound?
Ultrasound is defined as any sound wave above 20000Hz. Sound waves of this
frequency are above the human audible range and therefore cannot be heard
by humans. All sound waves, including ultrasound are longitudinal waves.
Medical ultrasounds are usually of the order of MEGAHERTZ (1-15MHz).
Ultrasound as all sound waves are caused by vibrations and therefore cause no
ionisation and are safe to use on pregnant women. Ultrasound is also able to
distinguish between muscle and blood and therefore show blood movement.
When an ultrasound wave meets a
boundary between two different
materials some of it is refracted and
some is reflected. The reflected wave
is detected by the ultrasound scanner
and forms the image.
Uses of Ultrasound
Obstetrics and Gynecology
The development and monitoring of a
developing foetus
http://www.createhealth.org/
http://archos.kewego.co.uk/video/iLyROoaft3bH.html
Cardiology
Seeing the inside of the heart to identify abnormal
structures or functions and measuring blood flow through
the heart and major blood vessels
Urology
•measuring blood flow through the kidney
•seeing kidney stones
•detecting prostate cancer early
The Piezoelectric Effect
piezoelectric means pressure electricity
Ultrasound waves are produced using the piezoelectric effect.
When a potential difference is applied across certain crystals (piezoelectric)
the crystals themselves deform and contract a little. If the p.d. applied is
alternating then the crystal vibrates at the same frequency and sends out
ultrasonic waves. For ultrasound - lead zirconate titanate (PZT) crystals are
used. This process also works in reverse. The piezoelectric crystal acts a
receiver of ultrasound by converting sound waves to alternating voltages and as
a transmitter by converting alternating voltages to sound waves
Discovered by
Pierre and Jacques
Curie in 1880.
The Transducer
The transducer probe is the main part of the ultrasound machine. The
transducer probe transmits and receives the ultrasound. The curved faceplate
shapes the ultrasound waves into a narrow beam.
Transducer probes come in many shapes and sizes. The shape of the probe
determines its field of view, and the frequency of emitted sound waves
(controlled by the tuning device) determines how deep the sound waves penetrate
and the resolution of the image. The ultrasound is pulsed. There must be a pause
to allow the reflected wave to be detected.
Ultrasound Equipment
Screen/Display
Transducer pulse
controls
Computer
Various Transducers
Portable
Why Ultrasound?
ADVANTAGES
•No known hazards – non ionizing
for patient and sonographer.
•Good for imaging soft tissue.
•Relatively cheap and portable.
DISADVANTAGES
•Cannot pass through bone
•Cannot pass through air spaces.
•Poor resolution.
Exam Style Question
a) Explain what an Ultrasound wave is.
(2 marks)
b) Describe how ultrasound images are carried out
(4 marks)
c) Discuss the uses, advantages and disadvantages of
Ultrasound in medicine.
(4 marks)
Acoustic Impedance, Z
As stated earlier, when an ultrasound wave meets a boundary between
two different materials some of it is refracted and some is reflected.
The reflected wave is detected by the ultrasound scanner and forms the
image. The proportion of the incident wave that is reflected depends on
the change in the acoustic impedance, Z.
Acoustic Impedance, Z of a medium is defined
as:
Z = c
Where  = the density of the material, kgm-3
c = speed of sound in that material, ms-1
TASK: What are the units of Z?
See page 201/203 of textbook for typical values
Exam Style Question
1) The acoustic impedance of a certain soft tissue is
1.63 x 106 kgm-2s-1 and its density is 1.09 x 103 kgm-3.
What is the speed of the ultrasound in this medium?
(2 marks)
2) The time base on a CRO was set to 50μscm-1. Reflected
pulses from either side of a fetal head are 2.4cm apart on
the screen. If the ultrasound travels at 1.5kms-1, calculate
the diameter of the fetal head.
(4 marks)
Intensity reflection
coefficient, 
At a boundary between mediums, the ratio of the intensity reflected, Ir to
the intensity incident, I0 is known as the intensity reflection coefficient, .
 = Ir
I0
The intensity of both the reflected and incident ultrasound waves depend on
the acoustic impedance, Z of the two mediums. Therefore the fraction of
the wave intensity reflected can be calculated for an ultrasound wave
travelling from medium 1, (acoustic impedance Z1) to medium 2 (acoustic
impedance Z2).
 = I r = Z2 - Z1
I0
Z2 + Z 1
2
If 2 mediums have a large
difference in impedance, then
most of the wave is reflected.
If they have a similar impedance
then none is reflected.
Impedance Matching / Gel
When ultrasound passes through two very
different materials the majority of it is
reflected. This happens between air and the
body, meaning that most ultrasound waves never
enter the body. To prevent this large difference
in impedance a coupling medium (gel) is used
between the air and the skin. The need to match
up similar impedances to ensure the waves pass
through the body is known as impedance
matching.
A-Scan
A-Scan (Amplitude scan)
• Gives no photo image
•Pulses of ultrasound sent into the body, reflected ultrasound is detected
and appear as vertical spikes on a CRO screen.
•The horizontal positions of the ‘spikes’ indicate the time it took for the
wave to be reflected.
•Commonly used to measure size of foetal head.
B-Scan
B-Scan (Brightness scan)
• An array of transducers are used and the ultrasound beam is spread out
across the body.
•Returning waves are detected and appear as spots of varying brightness.
•These spots of brightness are used to build up a picture.
The Doppler Effect
The apparent frequency
of a wave increases
when the source of a
wave is moving towards
you.
http://paws.kettering.edu/~drussell/Demos/doppler/carhorn.wav
The Doppler effect can be used to measure blood flow in adults, children
and developing babies.
Both the time for the reflected ultrasound wave and the ‘new’ frequency
of the reflected wave are measured. This enables the speed of blood flow
to be calculated. The greater the difference between the original
frequency and the reflected frequency, the greater the speed.
Computers then display this info as ‘moving images’ by updating data
several times per second.
Doppler Images
Red represents blood moving towards
the transducer (increased frequency).
Blue represents blood moving away from
the transducer (decreased frequency).
Cardiac B-scan
images with
Doppler scans
overlaid